Electric power converting device and electric power converting method

ABSTRACT

In one embodiment, an electric power converting device includes a converter which converts a three-phase AC voltage output from a three-phase AC power source, into a DC voltage of each phase of a three-phase AC load, and an inverter which converts the DC voltage converted by the converter, into a single-phase AC voltage of each phase of the three-phase AC load. The converter includes for each phase of an electric power system a circuit which consists of a plurality of switching elements connected in series. The electric power converting device controls on/off of a switching element corresponding to one of phases of the electric power system in the converter such that a voltage which reduces fluctuation of a DC voltage applied between the converter and the inverter and corresponding to each phase of the three-phase AC load is output from the converter for each phase of the electric power system.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon and claims benefit of priority from theJapanese Patent Application No. 2012-239216, filed on Oct. 30, 2012, theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to an electric powerconverting device and an electric power converting method.

BACKGROUND

An electric power converting device which outputs high electric powerconverts a high voltage. In this case, as a device which converts DCelectric power into AC electric power, a neutral-point-clamped (NPC)inverter is used. Further, as an inverter which outputs a high voltage,a single-phase NPC inverter which provides a full-bridge using two NPClegs per phase is used.

A DC link voltage of the above single-phase NPC inverter fluctuates at afrequency which is twice as a frequency of an output voltage. Further,when the single-phase NPC inverter outputs a low frequency voltage, afluctuation width of the DC link voltage which is a voltage of the DClink capacitor between an inverter and a converter increases.

To reduce this significant fluctuation, capacity of the DC linkcapacitor may be increased. However, when the capacity of the DC linkcapacitor is increased, it is not possible to avoid that a devicebecomes larger and more costly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view illustrating an example of a circuit configuration ofan electric power converting device according to an embodiment;

FIG. 2 is a view illustrating a table format of an example of arelationship between an output voltage of an inverter unit of theelectric power converting device and switching element states accordingto the embodiment;

FIG. 3 is a timing chart illustrating an example of a relationshipbetween an output voltage of the inverter unit of the electric powerconverting device and switching element states according to theembodiment;

FIG. 4 is a view illustrating a table format of an example of arelationship between an output voltage of a converter unit of theelectric power converting device and switching element states accordingto the embodiment;

FIG. 5 is a timing chart illustrating an example of a relationshipbetween an output voltage of the converter unit of the electric powerconverting device and switching element states according to theembodiment;

FIG. 6 is a flowchart illustrating an example of process of control upona power running operation of the converter unit of the power electricpower converting device according to the embodiment; and

FIG. 7 is a flowchart illustrating an example of process of control upona regenerative operation of the converter unit of the power electricpower converting device according to the embodiment.

DETAILED DESCRIPTION

In one embodiment, an electric power converting device includes aconverter which converts a three-phase AC voltage output from athree-phase AC power source, into a DC voltage of each phase of athree-phase AC load, an inverter which converts the DC voltage convertedby the converter, into a single-phase AC voltage of each phase of thethree-phase AC load, and a capacitor which is connected to a terminalbetween the converter and the inverter. The converter comprises for eachphase of an electric power system a circuit which consists of aplurality of switching elements connected in series. The electric powerconverting device further includes a control unit which controls on/offof a switching element corresponding to one of phases of the electricpower system in the converter such that a voltage which reducesfluctuation of a DC voltage applied between the converter and theinverter and corresponding to each phase of the three-phase AC load isoutput from the converter for each phase of the electric power system.

Hereinafter, an embodiment will be described with reference to thedrawings.

FIG. 1 is a view illustrating an example of a circuit configuration ofan electric power converting device according to the embodiment.

This electric power converting device converts a three-phase AC voltageE from a three-phase AC power source 1 into a DC voltage once. Further,the electric power converting device converts this DC voltage into anarbitrary AC voltage of an arbitrary frequency, and drives a three-phaseAC load. In the present embodiment, the three-phase AC load is athree-phase electric motor (MOT) 2. Further, in the present embodiment,three phases of an electric power system are referred to as R, S and Tphases, and three phases of the three-phase electric motor 2 arereferred to as U, V and W phases.

The electric power converting devices are classified into an electricpower converting device of the U phase, an electric power convertingdevice of the V phase and an electric power converting device of the Wphase. The electric power converting device of the U phase includes aconverter transformer TR_(U) which converts the three-phase AC voltage Einto a DC voltage, and a converter unit CNV_(U). Further, this electricpower converting device of the U phase includes an inverter unit INV_(U)which converts the DC voltage obtained by performing conversion in thisway, into a single-phase AC voltage.

Remarkable features of the present embodiment compared to an oldtechnique lie in controlling on/off of switching elements of converterunits and reducing fluctuation of DC link voltages. Hereinafter, wherenecessary, a DC link voltage is simply referred to as a DC voltage.

The converter unit CNV_(U) is a three-phase neutral-point-clamped (NPC)type. Further, this converter unit CNV_(U) employs a three-phasehalf-bridge configuration in which three NPC legs corresponding torespective phases of the R, S and T phases are connected in parallel.Output terminals of respective neutral points of the three NPC legs ofthe converter unit CNV_(U) are connected to a DC winding wire of theconverter transformer TR_(U). The converter unit CNV_(U) includes a DCvoltage terminal. This DC voltage terminal consists of a high potentialside terminal P_(U), a neutral point side terminal O_(U) and a lowpotential side terminal N_(U).

Further, the inverter unit INV_(U) is the same neutral-point-clamped(NPC) type as that of the converter unit CNV_(U). Furthermore, thisinverter unit INV_(U) employs a full-bridge configuration in which twoNPC legs are connected in parallel. The high potential side terminalP_(U), the neutral point side terminal O_(U) and the low potential sideterminal N_(U) are also common to those of the inverter unit INV_(U).

Further, two DC link capacitors C_(UP) and C_(UN) are provided betweenthe inverter unit INV_(U) and the converter unit CNV_(U).

One end of the DC link capacitor C_(UP) is connected to the highpotential side terminal P_(U). The other end of the DC link capacitorC_(UP) and one end of the DC link capacitor C_(UN) are connected to theneutral point side terminal O_(U). The other end of the DC linkcapacitor C_(UN) is connected to the low potential side terminal N_(U).

Similar to the electric power converting device of the U phase, theelectric power converting device of the V phase includes a convertertransformer TR_(V), a converter unit CNV_(V) and an inverter unitINV_(V). Configurations of the converter transformer TR_(V), theconverter unit CNV_(V) and the inverter unit INV_(V) are the same as theconfigurations of the converter transformer TR_(U), the converter unitCNV_(U) and the inverter unit INV_(U) of the electric power convertingdevice of the U phase.

Further, similar to the electric power converting devices of the U phaseand the V phase, the electric power converting device of the W phaseincludes a converter transformer TR_(W), a converter unit CNV_(W) and aninverter unit INV_(W). Configurations of the converter transformerTR_(w), the converter unit CNV_(w) and the inverter unit INV_(w) are thesame as the configurations of the converter transformer TR_(U), theconverter unit CNV_(U) and the inverter unit INV_(U) of the electricpower converting device of the U phase.

Further, voltage/electric current ratings of converter transformers,converter units and inverter units of the respective U, V and W phasesare the same between the respective phases.

Furthermore, as illustrated in FIG. 1, two DC link capacitors C_(VP) andC_(VN) are provided between the inverter unit INV_(V) and the converterunit CNV_(V) of the V phase.

One end of the DC link capacitor C_(VP) is connected to the highpotential side terminal P_(V). The other end of the DC link capacitorC_(VP) and one end of the DC link capacitor C_(VN) are connected to theneutral point side terminal O_(V). The other end of the DC linkcapacitor C_(VN) is connected to the low potential side terminal N_(V).

Further, as illustrated in FIG. 1, two DC link capacitors C_(WP) andC_(WN) are provided between the inverter unit INV_(W) and the converterunit CNV_(W) of the W phase.

One end of the DC link capacitor C_(WP) is connected to the highpotential side terminal P_(W). The other end of the DC link capacitorC_(WP) and one end of the DC link capacitor C_(WN) are connected to theneutral point side terminal O_(W). The other end of the DC linkcapacitor C_(WN) is connected to the low potential side terminal N_(W).

AC winding wires of the converter transformers TR_(U), TR_(V) and TR_(W)are connected in series in order of the converter transformers TR_(U),TR_(V) and TR_(W). Further, a converter transformer of a lowermost stageof the electric power converting device of each phase in FIG. 1 is theconverter transformer TR_(W). Furthermore, the converter transformerTR_(U) of the uppermost stage illustrated in FIG. 1 is connected to thethree-phase AC power source 1. According to this configuration, avoltage obtained by adding output voltages of the converter unitsCNV_(U), CNV_(V) and CNV_(W) of the respective phases to be output tothe electric power system is output the electric power system.

Further, one of two output terminals led from the inverter unit of eachphase to the three-phase electric motor 2 is mutually connected to oneof output terminals of inverter units of other phases. This connectionpoint is a virtual neutral point of the inverter unit of each phase. Theother output terminal led from the inverter unit of each phase to thethree-phase electric motor 2 is connected respectively to a terminal ofeach phase of the three phases of the three-phase electric motor 2.

Next, a detailed configuration of each inverter unit illustrated in FIG.1 will be described using the U phase as an example.

The inverter unit INV_(U) of the U phase illustrated in FIG. 1 includeseight switching elements S_(UA1), S_(UA2), S_(UA3), S_(UA4), S_(UB1),S_(UB2), S_(UB3) and S_(UB4). Further, this inverter unit INV_(U)includes eight freewheel diodes D_(UA1), D_(UA2), D_(UA3), D_(UA4),D_(UB1), D_(UB2), D_(UB3) and D_(UB4). These freewheel diodes areconnected to all switching elements by way of anti-parallel connectionon a one-on-one basis. Further, the inverter unit INV_(U) includes fourclamp diodes D_(UA5), D_(UA6), D_(UB5) and D_(UB6) connected to theneutral point.

These switching elements S_(UA1), S_(UA2), S_(UA3) and S_(UA4),freewheel diodes D_(UA1), D_(UA2), D_(UA3) and D_(UA4) and clamp diodesD_(UA5) and D_(UA6) form a first leg of the inverter unit INV_(U).

These switching elements S_(UA1), S_(UA2), S_(UA3) and S_(UA4) areconnected in series in order of S_(UA1), S_(UA2), S_(UA3) and S_(UA4)from the high potential side of the inverter unit INV_(U) to the lowpotential side. An anode of the clamp diode D_(UA5) is connected to theneutral point of the inverter unit INV_(U), and a cathode of the clampdiode D_(UA5) is connected to a connection point of the switchingelements S_(UA1) and S_(UA2). A cathode of the clamp diode D_(UA6) isconnected to the neutral point of the inverter unit INV_(U) side, and ananode of the clamp diode D_(UA6) is connected to a connection point ofthe switching elements S_(UA3) and S_(UA4).

The switching element S_(UA1) is connected with the freewheel diodeD_(UA1) by way of anti-parallel connection, and the switching elementS_(UA2) is connected with the freewheel diode D_(UA2) by way ofanti-parallel connection. Further, the switching element S_(UA3) isconnected with the freewheel diode D_(UA3) by way of anti-parallelconnection, and the switching element S_(UA4) is connected with thefreewheel diode D_(UA4) by way of anti-parallel connection.

Furthermore, the switching elements S_(UB1), S_(UB2), S_(UB3) andS_(UB4), the freewheel diodes D_(UB1), D_(UB2), D_(UB3) and D_(UB4) andthe clamp diodes D_(UB5) and D_(UB6) form a second leg of the inverterunit INV_(U).

These switching elements S_(UB1), S_(UB2), S_(UB3) and S_(UB4) areconnected in series in order of S_(UB1), S_(UB2), S_(UB3) and S_(UB4)from the high potential side of the inverter unit INV_(U) to the lowpotential side. An anode of the clamp diode D_(UB5) is connected to theneutral point of the inverter unit INV_(U) side. A cathode of the clampdiode D_(UB5) is connected to a connection point of the switchingelements S_(UB1) and S_(UB2) side. A cathode of the clamp diode D_(UB6)is connected to the neutral point of the inverter unit INV_(U) side. Ananode of the clamp diode D_(UB6) is connected to a connection point ofthe switching elements S_(UB3) and S_(UB4).

The switching element S_(UB1) is connected with the freewheel diodeD_(UB1) by way of anti-parallel connection, and the switching elementS_(UB2) is connected with the freewheel diode D_(UB2). Further, theswitching element S_(UB3) is connected with the freewheel diode D_(UB3)by way of anti-parallel connection, and the switching element S_(UB4) isconnected with the freewheel diode D_(UB4) by way of anti-parallelconnection.

That is, the inverter unit INV_(U) is an NPC full-bridge electric powerconverting device in which the switching elements S_(UA1), S_(UA2),S_(UA3) and S_(UA4) are connected in series and the switching elementsS_(UB1), S_(UB2), S_(UB3) and S_(UB4) are connected in series toconfigure the two legs.

Further, a potential difference V_(UA)−V_(UB) between a connection pointpotential V_(UA) of the switching elements S_(UA2) and S_(UA3) and aconnection point potential V_(UB) of the switching elements S_(UB2) andS_(UB3) is output to the three-phase electric motor 2. This potentialdifference means a PWM (Pulse Width Modulation) voltage.

Next, a detailed configuration of each converter unit illustrated inFIG. 1 will be described using the U phase as an example.

The converter unit CNV_(U) of the U phase includes twelve switchingelements S_(UR1), S_(UR2), S_(UR3), S_(UR4), S_(US1), S_(US2), S_(US3),S_(US4), S_(UT1), S_(UT2), S_(UT3) and S_(UT4). This converter unitCNV_(U) includes twelve freewheel diodes D_(UR1), D_(UR2), D_(UR3),D_(UR4), D_(US1), D_(US2), D_(US3), D_(US4), D_(UT1), D_(UT2), D_(UT3)and D_(UT4). These freewheel diodes are connected to all switchingelements by way of anti-parallel connection on a one-on-one basis.Further, the converter unit CNV_(U) includes six clamp diodes D_(UR5),D_(UR6), D_(US5), D_(US6), D_(UT5) and D_(UT6) connected to the neutralpoint of this converter unit CNV_(U) side.

More specifically, the switching elements S_(UR1), S_(UR2), S_(UR3) andS_(UR4), the freewheel diodes D_(UR1), D_(UR2), D_(UR3) and D_(UR4) andthe clamp diodes D_(UR5) and D_(UR6) configure the leg of the R phase ofthe converter unit CNV_(U).

The switching elements S_(UR1), S_(UR2), S_(UR3) and S_(UR4) areconnected in series in order of the switching elements S_(UR1), S_(UR2),S_(UR3) and S_(UR4) from the high potential side of the converter unitCNV_(U) to the low potential side.

An anode of the clamp diode D_(UR5) is connected to the neutral point ofthe converter unit CNV_(U) side. A cathode of the clamp diode D_(UR5) isconnected to a connection point of the switching elements S_(UR1) andS_(UR2). Further, a cathode of the clamp diode D_(UR6) is connected tothe neutral point of the converter unit CNV_(U) side. An anode of theclamp diode D_(UR6) is connected to a connection point of the switchingelements S_(UR3) and S_(UR4).

The switching element S_(UR1) is connected with the freewheel diodeD_(UR1) by way of anti-parallel connection, and the switching elementS_(UR2) is connected with the freewheel diode D_(UR2) by way ofanti-parallel connection. Further, the switching element S_(UR3) isconnected with the freewheel diode D_(UR3) by way of anti-parallelconnection, and the switching element S_(UR4) is connected with thefreewheel diode D_(UA4) by way of anti-parallel connection.

Furthermore, the switching elements S_(US1), S_(US2), S_(US3) andS_(US4), the freewheel diodes D_(US1), D_(US2), D_(US3) and D_(US4) andthe clamp diodes D_(US5) and D_(US6) configure a leg of the S phase ofthe converter unit CNV_(U). More specifically, the switching elementsS_(US1), S_(US2), S_(US3) and S_(US4) are connected in series in orderof the switching elements S_(US1), S_(US2), S_(US3) and S_(US4) from thehigh potential side of the converter unit CNV_(U) to the low potentialside.

An anode of the clamp diode D_(US5) is connected to the neutral point ofthe converter unit CNV_(U) side. A cathode of the clamp diode D_(UB5) isconnected to a connection point of the switching elements S_(US1) andS_(US2). A cathode of the clamp diode D_(US6) is connected to theneutral point of the converter unit CNV_(U) side. An anode of the clampdiode D_(US6) is connected to a connection point of the switchingelements S_(US3) and S_(US4).

The switching element S_(US1) is connected with the freewheel diodeD_(US1) by way of anti-parallel connection, and the switching elementS_(US2) is connected with the freewheel diode D_(US2) by way ofanti-parallel connection. Further, the switching element S_(US3) isconnected with the freewheel diode D_(US3) by way of anti-parallelconnection, and the switching element S_(US4) is connected with thefreewheel diode D_(US4) by way of anti-parallel connection.

Furthermore, the switching elements S_(UT1), S_(UT2), S_(UT3) andS_(UT4), the freewheel diodes D_(UT1), D_(UT2), D_(UT3) and D_(UT4) andthe clamp diodes D_(UT5) and D_(UT6) configure a leg of the T phase ofthe converter unit CNV_(U). More specifically, the switching elementsS_(UT1), S_(UT2), S_(UT3) and S_(UT4) are connected in series in orderof the switching elements S_(UT1), S_(UT2), S_(UT3) and S_(UT4) from thehigh potential side of the converter unit CNV_(U) to the low potentialside.

An anode of the clamp diode D_(UT5) is connected to the neutral point ofthe converter unit CNV_(U) side. A cathode of the clamp diode D_(UT5) isconnected to a connection point of the switching elements S_(UT1) andS_(UT2). Further, a cathode of the clamp diode D_(UT6) is connected tothe neutral point of the converter unit CNV_(U) side. An anode of theclamp diode D_(UT6) is connected to a connection point of the switchingelements S_(UT3) and S_(UT4).

The switching element S_(UT1) is connected with the freewheel diodeD_(UT1) by way of anti-parallel connection, and the switching elementS_(UT2) is connected with the freewheel diode D_(UT2) by way ofanti-parallel connection. The switching element S_(UT3) is connectedwith the freewheel diode D_(UT3) by way of anti-parallel connection, andthe switching element S_(UT4) is connected with the freewheel diodeD_(UT4) by way of anti-parallel connection.

That is, the converter unit CNV_(U) is a three-phase NPC electric powerconverting device in which the three legs are configured by connectingin series the switching elements S_(UR1), S_(UR2), S_(UR3) and S_(UR4)of the R phase, the switching elements S_(US1). S_(US2), S_(US3) andS_(US4) of the S phase and the switching elements S_(UT1), S_(UT2),S_(UT3) and S_(UT4) of the T phase.

In addition, a three-phase voltage consisting of a connection pointpotential V_(UR) of the R phase of the converter unit CNV_(U), aconnection point potential V_(US) of the leg of the S phase and theconnection point potential V_(UT) of the leg of the T phase is output toa DC winding wire of the converter transformer T_(RU).

The connection point potential V_(UR) is a connection point potential ofthe switching elements S_(UR2) and S_(UR3) of the leg of the R phase ofthe converter unit CNV_(U). The connection point potential V_(US) is aconnection point potential of the switching elements S_(US2) and S_(US3)of the leg of the S phase of the converter unit CNV_(U). Further, theconnection point potential V_(UT) is a connection point potential of theswitching elements S_(UT2) and S_(UT3) of the leg of the T phase of theconverter unit CNV_(U).

In the present embodiment, by connecting the converter unit CNV_(U) tothe DC winding wire of the converter transformer T_(RU) by Δ wireconnection, line voltages of V_(UR)−V_(US), V_(US)−V_(UT) andV_(UT)−V_(UR) of the three phases are output to the AC winding wire sideof the converter transformer T_(RU).

Configurations of the electric power converting device of the V phaseand the W phase are the same as that of the electric power convertingdevice of the U phase.

Although not illustrated in detail, the inverter unit INV_(V) of the Vphase includes eight switching elements S_(VA1), S_(VA2), S_(VA3),S_(VA4), S_(VB1), S_(VB2), S_(VB3) and S_(VB4). Further, this inverterunit INV_(V) includes eight freewheel diodes D_(VA1), D_(VA2), D_(VA3),D_(VA4), D_(VB1), D_(VB2), D_(VB3) and D_(VB4). Furthermore, theinverter unit INV_(V) includes four clamp diodes D_(VA5), D_(VA6),D_(VB5) and D_(VB6) connected to the neutral point.

Still further, the inverter unit INV_(W) of the W phase includes eightswitching elements S_(WA1), S_(WA2), S_(WA3), S_(WA4), S_(WB1), S_(WB2),S_(WB3) and S_(WB4). Moreover, this inverter unit INV_(V) includes eightfreewheel diodes D_(WA1), D_(WA2), D_(WA3), D_(WA4), D_(WB1), D_(WB2),D_(WB3) and D_(WB4) connected to all switching elements by way ofanti-parallel connection. Further, the inverter unit INV_(U) includesfour clamp diodes D_(WA5), D_(WA6), D_(WB5) and D_(WB6) connected to theneutral point.

Furthermore, the converter unit CNV_(V) of the V phase includes twelveswitching elements S_(VR1), S_(VR2), S_(VR3), S_(VR4), S_(VS1), S_(VS2),S_(VS3), S_(VS4), S_(VT1), S_(VT2), S_(VT3) and S_(VT4). This converterunit CNV_(V) includes twelve freewheel diodes D_(VR1), D_(VR2), D_(VR3),D_(VR4), D_(VS1), D_(VS2), D_(VS3), D_(VS4), D_(VT1), D_(VT2), D_(VT3)and D_(VT4). Further, the converter unit CNV_(U) includes six clampdiodes D_(VR5), D_(VR6), D_(VS5), D_(VS6), D_(VT5) and D_(VT6) connectedto the neutral point of this converter unit CNV_(U) side.

Furthermore, the converter unit CNV_(W) of the W phase includes twelveswitching elements S_(WR1), S_(WR2), S_(WR3), S_(WR4), S_(WS1), S_(WS2),S_(WS3), S_(WS4), S_(WT1), S_(WT2), S_(WT3) and S_(WT4). This converterunit CNV_(W) includes twelve freewheel diodes D_(WR1), D_(WR2), D_(WR3),D_(WR4), D_(WS1), D_(WS2), D_(WS3), D_(WS4), D_(WT1), D_(WT2), D_(WT3)and D_(WT4). Further, the converter unit CNV_(W) includes six clampdiodes D_(WR5), D_(WR6), D_(WS5), D_(WS6), D_(WT5) and D_(WT6) connectedto the neutral point of this converter unit CNV_(W) side.

An operation of the present embodiment employing the above-describedconfiguration will be described in detail.

First, a method of outputting a voltage from an inverter unit will bedescribed using the inverter unit INV_(U) of the U phase as an example.

The inverter unit INV_(U) employs a full-bridge configuration.Consequently, when a DC voltage of the inverter unit INV_(U) is V_(DC),the inverter unit INV_(U) can output DC voltages of five levelsconsisting of −V_(DC), −V_(DC)/2, 0, +V_(DC)/2 and +V_(DC).

Next, a method of driving the switching elements S_(UA1), S_(UA2),S_(UA3), S_(UA4), S_(UB1), S_(UB2), S_(UB3) and S_(UB4) of the inverterunit INV_(U) will be described.

In the present embodiment, a control device 10 is provided asillustrated in FIG. 1. This control device 10 includes an invertercontrol unit 11 which controls the inverter units and a convertercontrol unit 12 which controls the converter units.

The inverter control unit 11 controls on/off of the switching elementsof the inverter units.

The inverter control unit 11 controls on/off of the switching elementsS_(UA1), S_(UA2), S_(UA3), S_(UA4), S_(UB1), S_(UB2), S_(UB3) andS_(UB4), and the inverter unit INV_(U) outputs the potential differenceV_(UA)−V_(UB) to the three-phase electric motor 2. As this output, theabove-described voltages of five levels, that is, −V_(DC), −V_(DC)/2, 0,+V_(DC)/2 and +V_(DC) are applied to the three-phase electric motor 2.

FIG. 2 is a view illustrating a table format of an example of arelationship between output voltages of an inverter unit of the electricpower converting device and switching element states according to theembodiment.

FIG. 2 illustrates patterns of correspondence relationships betweenon/off states and output voltages (DC voltages) of the switchingelements S_(UA1), S_(UA2), S_(UA3), S_(UA4), S_(UB1), S_(UB2), S_(UB3)and S_(UB4) of the inverter unit INV_(U). The number of these patternsis nine.

Further, as illustrated in FIG. 2, when the switching element S_(UA1) ison, the switching element S_(UA3) is off. Furthermore, when theswitching element S_(UA2) is on, the switching element S_(UA4) is off.Still further, when the switching element S_(UB1) is on, the switchingelement S_(UB3) is off. Moreover, when the switching element S_(UB2) ison, the switching element S_(UB4) is off.

Thus, according to the present embodiment, the inverter control unit 11causes complementary operations of the switching elements S_(UA1) andS_(UA3) of the first leg in the inverter unit INV_(U), and causescomplementary operations of the switching elements S_(UA2) and S_(UA4)of the first leg in the inverter unit INV_(U). Thus, according to thepresent embodiment, the inverter control unit 11 causes complementaryoperations of the switching elements S_(UB1) and S_(UB3) of the secondleg in the inverter unit INV_(U), and causes complementary operations ofthe switching elements S_(UB2) and S_(UB4) of the second leg in theinverter unit INV_(U).

As illustrated in FIG. 2, there are three output patterns of 0 voltage.Further, there are two output patterns of −V_(DC)/2 and two outputpatterns of +V_(DC)/2. That is, output patterns of 0 voltage, −V_(DC)/2and +V_(DC)/2 have redundancy.

In the present embodiment, a method of outputting a PWM voltageV_(UA)−V_(UB) corresponding to an inverter U phase voltage command valueV_(U)* from the inverter control unit 11 of the control device 10 usingtriangle wave carrier modulation will be described.

FIG. 3 is a timing chart illustrating an example of a relationshipbetween an output voltage of the inverter unit of the electric powerconverting device and switching element states according to theembodiment.

The timing chart in FIG. 3 is a timing chart illustrating relationshipsbetween a state of a carrier modulated wave of the inverter unit INV_(U)and on/off states of the switching elements S_(UA1), S_(UA2), S_(UA3),S_(UA4), S_(UB1), S_(UB2), S_(UB3) and S_(UB4) of the inverter unitINV_(U).

The inverter control unit 11 generates two triangle waves CAR_(U1) andCAR_(U2) at a predetermined carrier frequency. Further, the invertercontrol unit 11 outputs voltage command values V_(UA)* and V_(UB)* ofthe inverter unit INV_(U). Furthermore, the inverter control unit 11compares the triangle waves CAR_(U1) and CAR_(U2) and the voltagecommand values V_(UA)* and V_(UB)* of the inverter unit INV_(U), andgenerates switching patterns of the eight switching elements S_(UA1),S_(UA2), S_(UA3), S_(UA4), S_(UB1), S_(UB2), S_(UB3) and S_(UB4) of theinverter unit INV_(U).

In addition, the voltage command value V_(UA)* matches the inverter Uphase voltage command value V_(U)*. Further, the voltage command valueV_(UB)* matches a value obtained by inverting the inverter U phasevoltage command value V_(U)*. That is, V_(UA)*=V_(U)* holds andV_(UB)*=−V_(U)* holds.

In the present embodiment, a maximum value of the voltage command valueV_(U)* is 1.0, and a minimum value of the voltage command value V_(U)*is −1.0. Then, a region of the voltage command value V_(U)* is supportedby two regions consisting of a region of a value of the triangle waveCAR_(U1) and a region of a value of the triangle CAR_(U2). In thepresent embodiment, a maximum value of the triangle wave CAR_(U1) is1.0, and a minimum value is 0.0. Further, the maximum value of thetriangle wave CAR_(U2) is 0.0, and a minimum value is −1.0.

FIG. 3 illustrates an example of operation states of the switchingelements S_(Um), S_(UA3), S_(UA2), S_(UA4), S_(UB1), S_(UB3), S_(UB2)and S_(UB4) when the voltage command V_(U)* is between 0.5 and 1.0.

Hereinafter, an operation of the switching element of the inverter unitINV_(U) with respect to each triangle wave will be described in detail.

The inverter control unit 11 turns on the switching element S_(UA1) andturns off the switching element S_(UA3) when the voltage command valueV_(UA)* is higher than the triangle wave CAR_(U1). The inverter controlunit 11 turns off the switching element S_(UA1) and turns on theswitching element S_(UA3) when the voltage command value V_(UA)* islower than the triangle wave CAR_(U1).

The inverter control unit 11 turns on the switching element S_(UA2) andturns off the switching element S_(UA4) when the voltage command valueV_(UA)* is higher than the triangle wave CAR_(U2). The inverter controlunit 11 turns off the switching element S_(UA2) and turns on theswitching element S_(UA4) when the voltage command value V_(UA)* islower than the triangle wave CAR_(U2).

Further, the inverter control unit 11 turns on the switching elementS_(UB1) and turns off the switching element S_(UB3) when the voltagecommand value V_(UB)* is higher than the triangle wave CAR_(U1). Theinverter control unit 11 turns off the switching element S_(UB1) andturns on the switching element S_(UB3) when the voltage command valueV_(UB)* is lower than the triangle wave CAR_(U1).

The inverter control unit 11 turns on the switching element S_(UB2) andturns off the switching element S_(UB4) when the voltage command valueV_(UB)* is higher than the triangle wave CAR_(U2). The inverter controlunit 11 turns off the switching element S_(UB2) and turns on theswitching element S_(UB4) when the voltage command value V_(UB)* islower than the triangle wave CAR_(U2).

More specifically, as indicated at a left end portion of the timingchart in FIG. 3, when the voltage command value V_(UA)* is higher thanthe triangle waves CAR_(U1) and CAR_(U2), and the voltage command valueV_(UB)* is lower than the triangle wave CAR_(U1) and is higher than thetriangle wave CAR_(U2), the inverter control unit 11 turns on theswitching elements S_(UA1), S_(UA2), S_(UB2) and S_(UB3) and turns offthe switching elements S_(UA3), S_(UA4), S_(UB1) and S_(UB4).

This switching pattern corresponds to a pattern “2” illustrated in FIG.2. Consequently, the output voltage is +V_(DC)/2. The patterns “1” to“3” illustrated in FIG. 2 appear when the voltage command value V_(U)*is higher than 0. Further, the patterns “7” to “9” illustrated in FIG. 2appear when the voltage command value V_(U)* is lower than 0.

According to the above operation, the inverter unit INV_(U) can outputthe PWM voltage V_(UA)−V_(UB) corresponding to the inverter U phasevoltage command value V_(U)*.

Further, a phase of the voltage command value for the inverter unitINV_(U), a phase of the voltage command value for the inverter unitINV_(V) and a phase of the voltage command value for the inverter unitINV_(W) are shifted 120 degrees, respectively.

Thus, except that the phases of the voltage command values are shifted,the operations of the inverter units INV_(U), INV_(V) and INV_(W) arecommon.

Next, a method of outputting a voltage from a converter unit will bedescribed using the converter unit CNV_(U) of the U phase as an example.A method of outputting a voltage from the converter unit CNV_(V) of theV phase and a method of outputting a voltage from the converter unit Ware the same as the method of outputting the voltage from the converterunit CNV_(U). This method of outputting the voltage from a converterunit indicates a remarkable feature of the present embodiment comparedto the old technique. By using this voltage outputting method, it ispossible to control on/off of switching elements of converter units, andreduce fluctuation of a DC link voltage.

The converter unit CNV_(U) employs the three-phase half-bridgeconfiguration, and therefore a method of outputting a voltage from eachphase of the electric power system will be described as a leg whichoutputs a R phase voltage of the system voltage.

The R phase leg of the converter unit CNV_(U) controls on/off of theswitching elements S_(UR1), S_(UR2), S_(UR3) and S_(UR4) and outputs thevoltage V_(UR). In this case, DC voltages are voltages of three levelsconsisting of −V_(DC)/2, 0 and +V_(DC)/2.

FIG. 4 is a view illustrating a table format of an example of arelationship between an output voltage of a converter unit of theelectric power converting device and switching element states accordingto the embodiment.

FIG. 4 illustrates on/off states of the switching elements S_(UR1),S_(UR2), S_(UR3) and S_(UR4) determined per output voltage. Asillustrated in FIG. 4, there are three patterns of these on/off states.

Further, in the present embodiment, the converter control unit 12 of thecontrol device 10 turns off the switching element S_(UR3) when turningon the switching element S_(UR1). The converter control unit 12 turns onthe switching element S_(UR3) when turning off the switching elementS_(UR1). Further, the converter control unit 12 turns off the switchingelement S_(UR4) when turning on the switching element S_(UR2). Theconverter control unit 12 turns on the switching element S_(UR4) whenturning off the switching element S_(UR2).

Thus, in the present embodiment, the converter control unit 12 causescomplementary operations of the switching elements S_(UR1) and S_(UR3)of the leg of the R phase of the converter unit CNV_(U), and causescomplementary operations of the switching elements S_(UR2) and S_(UR4)of the leg of the R phase of the converter unit CNV_(U).

Next, a method of outputting an R phase voltage V_(R) including theconverter units CNV_(U), CNV_(V) and CNV_(W) will be described. FIG. 5is a timing chart illustrating an example of a relationship between anoutput voltage of the converter unit of the electric power convertingdevice and switching element states according to the embodiment.

FIG. 5 illustrates a timing chart of the output voltages V_(UR), V_(VR)and V_(SR) of the R phases of the converter units CNV_(U), CNV_(V) andCNV_(W) of the respective phases, and the R phase voltage V_(R).

As described above, AC winding wires of the converter transformersTR_(W), TR_(V) and TR_(W) of the respective phases are connected inseries. Hence, the R phase voltage V_(R) is calculated based on theoutput voltages of the R phases of the converter units CNV_(U), CNV_(V)and CNV_(W) of the respective phases.

When a winding wire ratio of the converter transformers TR_(U), TR_(V)and TR_(W) is DC winding wire:AC winding wire=1:N,V_(R)=N×(V_(UR)+V_(VR)+V_(WR)) holds.

Points of time at which below operations a to i are performed correspondto symbols a to i described in FIG. 5.

For example, the point of time at which the operation a is performed isa point of time of the symbol a illustrated in FIG. 5. Further, methodsof performing operations upon a power running operation and aregenerative operation will be separately described.

First, an operation of performing the power running operation thatelectric power flows in from the electric power system will be describedwith reference to FIG. 6.

First, the determining unit 12 a of the converter control unit 12 of thecontrol device 10 acquires all DC voltages V_(DCUP), V_(DCUN), V_(DCVP),V_(DCVN), V_(DCWP) and V_(DCWN) corresponding to respective voltages ofDC link capacitors of the converter units CNV_(U), CNV_(V) and CNV_(W)of the respective phases. The determining unit 12 a specifies a minimumDC voltage among these DC voltages (step S1). This DC voltage is avoltage of the most significant fluctuation compared to an average ofthe respective phases. This specifying time is a timing of a in FIG. 5.

All DC voltages are the voltage V_(DCUP) of the DC link capacitor C_(UP)of the converter unit CNV_(U) side, the voltage V_(DCUN) of the DC linkcapacitor C_(UN) of the converter unit CNV_(U) side, the voltageV_(DCVP) of the DC link capacitor C_(VP) of the converter unit CNV_(V),the voltage V_(DCVN) of the DC link capacitor C_(VN) of the converterunit CNV_(V) side, the voltage V_(DCWP) of the DC link capacitor C_(WP)of the converter unit CNV_(W) side, and the voltage V_(DCWN) of the DClink capacitor C_(WN) of the converter unit CNV_(W) side.

In the present embodiment, the voltage V_(DCUP) of the DC link capacitorC_(UP) of the converter unit CNV_(U) side of the U phase is a minimumvalue.

In this case, it is necessary to make the voltage V_(DCUP) close to anaverage value of all DC voltages by causing an inflow of electric powerfrom the converter unit CNV_(U) to the DC link capacitor C_(UP) of the Uphase. Further, the inflow electric power is represented by a product ofan output voltage and an electric current. Consequently, the converterunit CNV_(U) needs to output as high a voltage as possible to the DClink capacitor side. That is, as illustrated in FIG. 5, the phaseselecting unit 12 b of the converter control unit 12 selects a risingphase α₁ of the largest voltage width as a phase of the output voltageV_(UR) of the R phase from the converter unit CNV_(U). This voltagewidth means duration of time from a rising to a falling in the timingchart illustrated in FIG. 5. The converter control unit 12 controlson/off of the switching elements S_(UR1), S_(UR2), S_(UR3) and S_(UR4)such that a waveform of the output voltage V_(UR) becomes a waveformcorresponding to this selected phase α₁ (step S2). The switching patternin this case corresponds to the pattern “1” illustrated in FIG. 4. Inthis case, a DC voltage from the converter unit is +V_(DC)/2 asillustrated in FIG. 4. This operation is the operation b in FIG. 5.

Further, the determining unit 12 a of the converter control unit 12specifies a minimum DC voltage among the DC voltages V_(DCVP), V_(DCVN),V_(DCWP) and V_(DCWN) of the V phase and the W phase other than this Uphase (step S3). This minimum DC voltage is a voltage of the mostsignificant fluctuation compared to an average of the respective phasesamong the DC voltages V_(DCVP), V_(DCVN), V_(DCWP) and V_(DCWN). Thisoperation is the operation c in FIG. 5. In the present embodiment, theDC voltage V_(DCVP) of the DC link capacitor C_(VP) of the converterunit CNV_(V) side of the V phase is a minimum value.

In this case, it is necessary to make the DC voltage V_(DCVP) of the DClink capacitor C_(VP) of the V phase close to an average value bycausing an inflow of electric power from the converter unit CNV_(U) tothe DC link capacitor C_(VP) of the V phase. As described above, theelectric power which flows in the converter unit is represented by aproduct of an output voltage and an electric current. Consequently, theconverter unit CNV_(U) needs to output as high a voltage as possible tothe DC link capacitor side. That is, as illustrated in FIG. 5, the phaseselecting unit 12 b of the converter control unit 12 selects a risingphase α₂ of the second largest voltage width as a phase of the outputvoltage V_(VR) of the R phase from the converter unit CNV_(U). Theconverter control unit 12 controls on/off of the switching elementsS_(VR1), S_(VR2), S_(VR3) and S_(VR4) such that a waveform of the outputvoltage V_(VR) becomes a waveform corresponding to this selected phaseα₂ (step S4). The switching pattern in this case corresponds to thepattern “1” illustrated in FIG. 4. This operation is the operation d inFIG. 5.

Further, the converter control unit 12 controls on/off of the switchingelements S_(WR1), S_(WR2), S_(WR3) and S_(WR4) such that the voltage isnot output from the converter unit CNV_(W) of the remaining W phase(step S5). The switching pattern in this case corresponds to the pattern“2” illustrated in FIG. 4. This operation is the operation e in FIG. 5.That is, 0 voltage is output from the converter unit CNV_(W), andtherefore electric power does not flow in the DC link capacitors C_(WP)and C_(WN) of the converter unit CNV_(W) side.

Further, the determining unit 12 a of the converter control unit 12specifies a maximum DC voltage among the DC voltages V_(DCUP), V_(DCUN),V_(DCUP) and V_(DCUN) of the U phase and the V phase other than this Wphase (step S6). This maximum DC voltage is a voltage of the mostsignificant fluctuation compared to an average of the respective phasesamong the DC voltages V_(DCUP), V_(DCUN), V_(DCVP) and V_(DCVN). Thisspecifying time is a timing of f in FIG. 5. In the present embodiment,the DC voltage V_(DCUP) is a maximum value.

In this case, it is necessary to stop an inflow of electric power fromthe electric power system to the converter unit CNV_(U), and make the DCvoltage V_(DCUP) of the DC link capacitor C_(UP) of the U phase close toan average value. Hence, the phase selecting unit 12 b of the convertercontrol unit 12 selects a falling phase π−α₂ of a small voltage width asthe phase of the output voltage V_(UR) of the R phase from the converterunit CNV_(U) such that an output of the voltage from the converter unitCNV_(U) stops. The converter control unit 12 controls on/off of theswitching elements S_(UR1), S_(UR2), S_(UR3) and S_(UR4) such that awaveform of the output voltage V_(UR) becomes a waveform correspondingto this selected phase (step S7). The switching pattern in this casecorresponds to the pattern “2” illustrated in FIG. 4. This operation isthe operation g in FIG. 5.

Further, the phase selecting unit 12 b of the converter control unit 12selects a falling phase π−α₁ of a large voltage width as the phase ofthe output voltage V_(VR) of the R phase from the converter unit CNV_(U)such that an output of the voltage from the converter unit CNV_(U)stops. The converter control unit 12 controls on/off of the switchingelements S_(VR1), S_(VR2), S_(VR3) and S_(VR4) such that a waveform ofthe output voltage V_(VR) becomes a waveform corresponding to thisselected phase (step S8). The switching pattern in this case correspondsto the pattern “2” illustrated in FIG. 4. This operation is theoperation h in FIG. 5.

The converter control unit 12 controls the switching elements of theconverter unit of each phase such that the same voltage as a voltage ina positive voltage output period is output from the converter unit ofeach phase to prevent bias magnetism of a transformer in a negativevoltage output period. This operation is the operation i in FIG. 5.

Next, an operation of performing the regenerative operation thatelectric power flows out to the electric power system will be describedwith reference to FIG. 7.

First, the determining unit 12 a of the converter control unit 12 of thecontrol device 10 acquires all DC voltages V_(DCUP), V_(DCUN), V_(DCVP),V_(DCVN), V_(DCWP) and V_(DCWN) corresponding to respective voltages ofDC link capacitors of the converter units CNV_(U), CNV_(V) and CNV_(W)of the respective phases. The determining unit 12 a specifies a maximumDC voltage among these voltages (step S11). This DC voltage is a voltageof the most significant fluctuation compared to an average of therespective phases. In the present embodiment, the voltage V_(DCUP) ofthe DC link capacitor C_(UP) of the converter unit CNV_(U) side of the Uphase is a maximum value.

In this case, it is necessary to make the DC voltage V_(DCUP) of the DClink capacitor C_(UP) of the U phase close to an average value of all DCvoltages by causing an outflow of electric power from the converter unitCNV_(U) side to the electric power system. Further, the electric powerwhich flows out from converter unit side to the electric power system isrepresented by a product of an output voltage and an electric current.Consequently, the converter unit CNV_(U) needs to output as high avoltage as possible to the electric power system. That is, asillustrated in FIG. 5, the phase selecting unit 12 b of the convertercontrol unit 12 selects a rising phase α₁ of the largest voltage widthas a phase of the output voltage V_(AR) of the R phase from theconverter unit CNV_(U). The converter control unit 12 controls on/off ofthe switching elements S_(UR1), S_(UR2), S_(UR3) and S_(UR4) such that awaveform of the output voltage V_(UR) becomes a waveform correspondingto this selected phase α₁ (step S12). The switching pattern in this casecorresponds to the pattern “3” illustrated in FIG. 4. In this case, a DCvoltage from the converter unit is −V_(DC)/2 as illustrated in FIG. 4.

Further, the determining unit 12 a of the converter control unit 12specifies a maximum DC voltage among the DC voltages V_(DCVP), V_(DCVN),V_(DCWP) and V_(DCWN) of the V phase and the W phase other than this Uphase (step S13). This maximum DC voltage is a voltage of the mostsignificant fluctuation compared to an average of the respective phasesamong the DC voltages V_(DCVP), V_(DCVN), V_(DCWP) and V_(DCWN). In thepresent embodiment, the DC voltage V_(DCVP) of the converter unitCNV_(V) side of the V phase is a maximum value.

In this case, it is necessary to make the DC voltage V_(DCVP) of the DClink capacitor C_(VP) of the V phase close to an average value bycausing an outflow of electric power from the converter unit CNV_(V)side to the electric power system. Further, as described above, theelectric power which flows out from converter unit side to the electricpower system is represented by a product of an output voltage and anelectric current. Consequently, the converter unit CNV_(V) needs tooutput as high a voltage as possible to the electric power system. Thatis, as illustrated in FIG. 5, the phase selecting unit 12 b of theconverter control unit 12 selects a rising phase α₂ of the secondlargest voltage width as a phase of the output voltage V_(VR) of the Rphase from the converter unit CNV_(V). The converter control unit 12controls on/off of the switching elements S_(VR1), S_(VR2), S_(UR3) andS_(VR4) such that a waveform of the output voltage V_(VR) becomes awaveform corresponding to this selected phase α₂ (step S14). Theswitching pattern in this case corresponds to the pattern “3”illustrated in FIG. 4.

Further, the converter control unit 12 controls on/off of the switchingelements S_(WR1), S_(WR2), S_(WR3) and S_(WR4) such that the voltage isnot output from the converter unit CNV_(W) of the remaining W phase tothe electric power system (step S15). The switching pattern in this casecorresponds to the pattern “2” illustrated in FIG. 4. That is, 0 voltageis output from the converter unit CNV_(W), and therefore electric powerdoes not flow in the DC link capacitors C_(WP) and C_(WN) of theconverter unit CNV_(W) side.

Further, the determining unit 12 a of the converter control unit 12specifies a minimum DC voltage among the DC voltages V_(DCUP), V_(DCUN),V_(DCUP) and V_(DCUN) of the U phase and the V phase other than this Wphase (step S16). This minimum DC voltage is a voltage of the mostsignificant fluctuation compared to an average of the respective phasesamong the DC voltages V_(DCUP), V_(DCUN), V_(DCVP) and V_(DCVN). In thepresent embodiment, the DC voltage V_(DCUP) of the converter unitCNV_(U) side of the U phase is a minimum value.

In this case, it is necessary to stop an outflow of electric power fromthe converter unit CNV_(U) to the electric power system, and make the DCvoltage V_(DCUP) of the DC link capacitor C_(UP) of the U phase close toan average value. Hence, the phase selecting unit 12 b of the convertercontrol unit 12 selects a falling phase π−α₂ of a small voltage width asthe phase of the output voltage V_(UR) of the R phase from the converterunit CNV_(U) such that an output of the voltage from the converter unitCNV_(U) stops. The converter control unit 12 controls on/off of theswitching elements S_(UR1), S_(UR2), S_(UR3) and S_(UR4) such that awaveform of the output voltage V_(UR) becomes a waveform correspondingto this phase (step S17). The switching pattern in this case correspondsto the pattern “2” illustrated in FIG. 4.

Further, the phase selecting unit 12 b of the converter control unit 12selects a falling phase π−α₁ of a large voltage width as the phase ofthe output voltage V_(VR) of the R phase from the converter unit CNV_(U)such that an output of the voltage from the converter unit CNV_(U)stops. The converter control unit 12 controls on/off of the switchingelements S_(VR1), S_(VR2), S_(VR3) and S_(VR4) such that a waveform ofthe output voltage V_(VR) becomes a waveform corresponding to this phase(step S18). The switching pattern in this case corresponds to thepattern “2” illustrated in FIG. 4.

The converter control unit 12 controls the switching elements of theconverter unit of each phase such that the same voltage as a voltage ina positive voltage output period is output from the converter unit ofeach phase to prevent bias magnetism of a transformer in a negativevoltage output period.

The rising phases α₁ and α₂ of the voltage are determined according toan output voltage amplitude. The R phase voltage V_(R) output from theconverter unit of each phase illustrated in FIG. 1 is expressed byfollowing equation (1) by expanding Fourier series.

$\begin{matrix}{V_{R} = {\frac{8\;{NV}_{DC}}{n\;\pi}{\sum\limits_{{n = 1},3,{5\mspace{11mu}\ldots}}^{\infty}\;{\left\{ {{\cos\left( {n\;\alpha_{1}} \right)} + {\cos\left( {n\;\alpha_{2}} \right)}} \right\}{\sin\left( {n\;\omega\; t} \right)}}}}} & {{Equation}\mspace{14mu}(1)}\end{matrix}$

Wherein n is a harmonic order, and is a fundamental wave voltage whenn=1 holds. That is, the fundamental wave of the R phase voltage V_(R) isexpressed by following equation (2).

$\begin{matrix}{V_{R} = {\frac{8\;{NV}_{DC}}{n\;\pi}\left\{ {{\cos\left( \alpha_{1} \right)} + {\cos\left( \alpha_{2} \right)}} \right\}{\sin\left( {\omega\; t} \right)}}} & {{Equation}\mspace{14mu}(2)}\end{matrix}$

When the voltage amplitude is controlled to satisfy a given voltage userate M (when a voltage peak value is 2V_(DC), M=1 holds), followingequation (3) needs to be satisfied.

$\begin{matrix}{{4\frac{{\cos\left( \alpha_{1} \right)} + {\cos\left( \alpha_{2} \right)}}{\pi}} = M} & {{Equation}\mspace{14mu} 3}\end{matrix}$

Thus, by changing the rising phases α₁ and α₂ of the voltage by theconverter control unit 12 of the control device 10, it is possible tooutput the output voltage amplitude from the converter unit as anarbitrary value and control input/output electric power related to theconverter unit. Further, by taking into account a difference betweenrising phases of the voltage from the converter unit, a harmonic of aspecific order is reduced. When, for example, following equation (4) issatisfied, fifth and seventh-order harmonics can be reduced.α₁−α₂=π/6  Equation (4)

That is, in the present embodiment, the rising phases α₁ and α₂ of thevoltage which simultaneously satisfies equation (3) and equation (4) arecalculated in advance according to the voltage use rate M expressed byequation (3). The converter control unit 12 of the control device 10selects one of the rising phases α₁ and α₂ as a phase of the outputvoltage of each phase from the converter unit according to whether a DCvoltage is high or low. By selecting the rising phase in this way, it ispossible to output a converter voltage which simultaneously satisfiesthe voltage amplitude and reduction of a harmonic.

The above-described example where, in step S5 upon the power runningoperation, on/off of switching elements is controlled such that aconverter unit of one remaining converter unit does not output a voltageafter selecting the rising phases α₁ and α₂ of the voltage has beendescribed. However, the present invention is not limited to this, andthe converter control unit 12 of the control device 10 may select therising phase α₃ after selecting the rising phases α₁ and α₂ of thevoltage, and control on/off of switching elements of one remaining phasesuch that a waveform becomes a waveform matching this phase.

Thus, when selecting the rising phase α₃, the determining unit 12 a ofthe converter control unit 12 specifies a maximum DC voltage among theDC voltages V_(DCWP), V_(DCUN), V_(DCVP), V_(DCVN), V_(DCWP) andV_(DCWN) of the respective phases instead of step S6. Further, the phaseselecting unit 12 b of the converter control unit 12 selects the fallingphase π−α₃ as the phase of the output voltage of the R phase from aconverter unit such that an output of the voltage from the converterunit of a phase related to this specified voltage stops. The convertercontrol unit 12 controls on/off of the switching elements such that awaveform becomes a waveform corresponding to this phase.

Further, the above-described example where, in step S15 upon theregenerative operation, on/off of switching elements is controlled suchthat a converter unit of one remaining converter unit does not output avoltage after selecting the rising phases α₁ and α₂ of the voltage hasbeen described. However, the present invention is not limited to this,and the converter control unit 12 of the control device 10 may selectthe rising phase α₃ after selecting the rising phases α₁ and α₂ of thevoltage, and control on/off of switching elements of one remaining phasesuch that a waveform becomes a waveform matching this phase.

Thus, when selecting the rising phase α₃, the determining unit 12 a ofthe converter control unit 12 specifies a minimum DC voltage among theDC voltages V_(DCUP), V_(DCUN), V_(DCUP), V_(DCVN), V_(DCWP) andV_(DCWN) of the respective phases instead of step S16. Further, thephase selecting unit 12 b of the converter control unit 12 selects thefalling phase π−α₃ as the phase of the output voltage of the R phasefrom a converter unit such that an output of the voltage from theconverter unit of a phase related to this specified voltage stops. Theconverter control unit 12 controls on/off of the switching elements suchthat a waveform becomes a waveform corresponding to this phase.

The rising phases α₁, α₂ and α₃ of the voltage are determined accordingto an output voltage amplitude. The R phase voltage V_(R) output fromthe converter in FIG. 1 is expressed by following equation (5) byexpanding Fourier series.

$\begin{matrix}{V_{R} = {\frac{12\;{NV}_{DC}}{n\;\pi}{\sum\limits_{{n = 1},3,{5\mspace{11mu}\ldots}}^{\infty}\;{\left\{ {{\cos\left( {n\;\alpha_{1}} \right)} + {\cos\left( {n\;\alpha_{2}} \right)} + {\cos\left( {n\;\alpha_{3}} \right)}} \right\}{\sin\left( {n\;\omega\; t} \right)}}}}} & {{Equation}\mspace{14mu} 5}\end{matrix}$

Wherein n is a harmonic order, and is a fundamental wave voltage whenn=1 holds. That is, the fundamental wave of the R phase voltage V_(R) isexpressed by following equation (6).

$\begin{matrix}{V_{R} = {\frac{12\;{NV}_{DC}}{n\;\pi}\;\left\{ {{\cos\left( \alpha_{1} \right)} + {\cos\left( \alpha_{2} \right)} + {\cos\left( {n\;\alpha_{3}} \right)}} \right\}{\sin\left( {\omega\; t} \right)}}} & {{Equation}\mspace{14mu} 6}\end{matrix}$

When the voltage amplitude is controlled to satisfy a given voltage userate M (when a voltage peak value is 3V_(DC), M=1 holds), followingequation (7) needs to be satisfied.

$\begin{matrix}{{4\frac{{\cos\left( \alpha_{1} \right)} + {\cos\left( \alpha_{2} \right)} + {\cos\left( \alpha_{3} \right)}}{\pi}} = M} & {{Equation}\mspace{14mu} 7}\end{matrix}$

Thus, by changing the rising phases α₁, α₂ and α₃ of the voltage by theconverter control unit 12, it is possible to output the output voltageamplitude as an arbitrary value and control input/output electric powerrelated to the converter. Further, by taking into account a differencebetween rising phases of the voltage, a harmonic of a specific order isreduced. When, for example, following equation (8) and equation (9) aresatisfied, it is possible to make the fifth and seventh-order harmonic 0theoretically.cos(5α₁)+cos(5α₂)+cos(5α₃)=0  Equation (8)cos(7α₁)+cos(7α₂)+cos(7α₃)=0  Equation (9)

Similar to the voltage outputting method described above, it is possibleto output a S phase voltage V_(S) including the converter units CNV_(U),CNV_(V) and CNV_(W) and output a T phase voltage V_(T) including theconverter units CNV_(U), CNV_(V) and CNV_(W).

As described above, the electric power converting device according tothe present embodiment controls on/off of switching elements of aconverter such that a voltage which reduces fluctuation of a DC voltagecorresponding to each phase of a three-phase AC load is output from theconverter. By this means, in a situation in which an inverter outputs alow frequency voltage, it is possible to reduce fluctuation of a DC linkvoltage without increasing a capacity value of the DC link capacitor.Consequently, it is possible to achieve reduction of fluctuation of theDC link voltage, and then reduce a capacity value of the DC linkcapacitor.

According to the present embodiment, one pulse control is adopted tocontrol converter units. This one pulse control refers to switchingcontrol which is performed once at one cycle of a voltage of each phasefrom a converter unit. Consequently, a voltage rising phase of aconverter unit of each phase is classified into 0, α₁ and α₂ accordingto each phase of the U, V and W phases, so that it is possible toprovide a degree of freedom of the voltage width of an output voltage ofthe converter unit of each phase.

According to the operation of the present embodiment, it is possible tocontrol electric power which flows in from the electric power system tothe converter unit of each phase and, consequently, reduce fluctuationof the DC link voltage. Further, it is possible to classify voltagerising phases into a plurality of phases, and, consequently, it ispossible to both control an electric current which flows in theconverter unit and reduce a harmonic by adjusting a voltage amplitude.

As a fringe effect, by adopting one pulse control, it is possible tocontain the number of times of switching of converter units at minimum.Consequently, loss produced by switching elements of a converter unitbecomes little, so that it is possible to make a cooler of the electricpower converting device smaller.

Further, according to the present embodiment, the electric powerconverting device extracts a maximum value or a minimum value of all DCvoltages V_(DCUP), V_(DCUN), V_(DCVP), V_(DCVN), V_(DCVN), V_(DCWP) andV_(DCWN) corresponding to respective voltages of the DC link capacitorsof the converter units CNV_(U), CNV_(V), CNV_(W) of the respectivephases, and preferentially compensates for electric power such that theextracted value becomes close to an average value of the respectivephases. Consequently, it is possible to minimize fluctuation of a DCvoltage.

Further, according to the present embodiment, the converter control unit12 targets at suppressing fluctuation of the DC voltages V_(DCUP),V_(DCUN), V_(DCVP), V_(DCVN), V_(DCWP) and V_(DCWN) of the DC linkcapacitors segmented at the respective neutral points on the converterunits CNV_(U) side, CNV_(V) side and CNV_(W) side of the respectivephases. Consequently, it is also possible to suppress fluctuation ofneutral point potentials of converter units.

As described above, according to the present embodiment, the convertercontrol unit 12 targets at suppressing fluctuation of DC voltagessegmented at the neutral points of the converter units CNV_(U) side,CNV_(V) side and CNV_(W) side of the respective phases. However, insteadof this, the converter unit may target at suppressing fluctuation ofthree DC voltages including DC voltages of two DC link capacitors of theU phase, DC voltages of two DC link capacitors of the V phase and DCvoltages of two DC link capacitors of the W phase without beingsegmented at neutral points.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel methods and systems describedherein may be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the methods andsystems described herein may be made without departing from the spiritof the inventions. The accompanying claims and their equivalents areintended to cover such forms or modifications as would fall within thescope and spirit of the inventions.

What is claimed is:
 1. An electric power converting device comprising: aconverter which converts a three-phase AC voltage output from athree-phase AC power source, into a DC voltage of each phase of athree-phase AC load; an inverter which comprises a plurality ofneutral-point-clamped (NPC) legs per phase and converts the DC voltageconverted by the converter, into a single-phase AC voltage of each phaseof the three-phase AC load; and a capacitor which is connected to aterminal between the converter and the inverter, wherein: the convertercomprises for each phase of an electric power system a circuit whichconsists of a plurality of switching elements connected in series; andthe electric power converting device further comprises a control unitwhich controls on/off of a switching element corresponding to one ofphases of the electric power system in the converter such that a voltagewhich reduces fluctuation of a DC voltage applied between the converterand the inverter and corresponding to each phase of the three-phase ACload is output from the converter for each phase of the electric powersystem.
 2. The electric power converting device according to claim 1,wherein the control unit specifies a voltage of maximum fluctuationcompared to an average of each phase among DC voltages applied betweenthe converter and the inverter and corresponding to the each phase ofthe three-phase AC load, specifies a phase related to the voltage, andcontrols on/off of a switching element corresponding to the specifiedphase of the converter such that a voltage which reduces the fluctuationis output from the converter; and specifies a voltage of maximumfluctuation other than the specified phase and among DC voltages of theeach phase of the three-phase AC load, specifies a phase related to thevoltage, and controls on/off of a switching element corresponding to thespecified phase of the converter such that a voltage which reduces thefluctuation is output from the converter.
 3. The electric powerconverting device according to claim 1, wherein the control unit uponpower running, specifies a minimum voltage among DC voltages appliedbetween the converter and the inverter and corresponding to the eachphase of the three-phase AC load, specifies a phase related to thevoltage, and controls on/off of a switching element corresponding to thespecified phase of the converter such that a voltage which increases thevoltage is output from the converter and specifies a minimum voltageamong DC voltages of the each phase of the three-phase AC load otherthan that of the specified phase, specifies a phase related to thevoltage, and controls on/off of a switching element corresponding to thespecified phase of the converter such that a voltage which increases thevoltage is output from the converter, and upon regeneration, specifies amaximum voltage among the DC voltages of the each phase of thethree-phase AC load between the converter and the inverter, specifies aphase related to the voltage, and controls on/off of a switching elementcorresponding to the specified phase of the converter such that avoltage which reduces the voltage is output from the converter; andspecifies a maximum voltage among DC voltages of the each phase of thethree-phase AC load other than the specified phase, specifies a phaserelated to the voltage, and controls on/off of a switching elementcorresponding to the specified phase of the converter such that avoltage which reduces the voltage is output from the converter.
 4. Theelectric power converting device according to claim 1, wherein: theconverter comprises for each phase of an electric power system a circuitwhich consists of a plurality of switching elements connected in seriesand segmented at a neutral point; and the control unit controls, foreach phase of the electric power system, on/off of a switching elementcorresponding to one of phases of the electric power system of theconverter such that a voltage which reduces fluctuation of six DCvoltages applied between the converter and the inverter and segmented ona high potential side and a low potential side at the neutral pointcorresponding to each phase of the three-phase AC load is output fromthe converter.
 5. The electric power converting device according toclaim 1, wherein the control unit: upon power running, specifies aminimum voltage among DC voltages applied between the converter and theinverter and corresponding to the each phase of the three-phase AC load,specifies a phase related to the voltage, and controls on/off of aswitching element corresponding to the specified phase of the convertersuch that a voltage which increases the voltage is output from theconverter, specifies a minimum voltage among DC voltages of the eachphase of the three-phase AC load other than that of the specified phase,specifies a phase related to the voltage, and controls on/off of aswitching element corresponding to the specified phase of the convertersuch that a voltage which increases the voltage is output from theconverter, and controls on/off of a switching element corresponding to aphase of the converter which is not specified such that a DC voltage ofthe phase which is not specified is not output from the converter; andupon regeneration, specifies a maximum voltage among the DC voltages ofthe each phase of the three-phase AC load between the converter and theinverter, specifies a phase related to the voltage, and controls on/offof a switching element corresponding to the specified phase of theconverter such that a voltage which reduces the voltage is output fromthe converter, specifies a maximum voltage among DC voltages of the eachphase of the three-phase AC load other than that of the specified phase,specifies a phase related to the voltage, and controls on/off of aswitching element corresponding to the specified phase of the convertersuch that a voltage which reduces the voltage is output from theconverter, and controls on/off control of the switching elementcorresponding to the phase of the converter which is not specified suchthat the DC voltage of the phase which is not specified is not outputfrom the converter.
 6. An electric power converting method used for anelectric power converting device which comprises: a converter whichconverts a three-phase AC voltage output from a three-phase AC powersource, into a DC voltage of each phase of a three-phase AC load; aninverter which comprises a plurality of neutral-point-clamped (NPC) legsper phase and converts the DC voltage converted by the converter, into asingle-phase AC voltage of each phase of the three-phase AC load; and acapacitor which is connected to a terminal between the converter and theinverter, and wherein the converter comprises for each phase of anelectric power system a circuit which consists of a plurality ofswitching elements connected in series, the electric power convertingmethod comprising: controlling, for each phase of the electric powersystem, on/off of a switching element corresponding to one of phases ofthe electric power system in the converter such that a voltage whichreduces fluctuation of a DC voltage applied between the converter andthe inverter and corresponding to each phase of the three-phase AC loadis output from the converter.